A medical institution is equipped with a radiographic apparatus configured to obtain a subject image with radiation. Such a radiographic apparatus suppresses a dose of radiation to be emitted to the extent possible upon radiography. This is because radiation exposure unnecessary for a subject has to be avoided.
Accordingly, a granular false image referred to as statistical noise likely appears in the obtained image. The statistical noise is, for example, derived from unevenness of radiation detection by each detecting element of a radiation detector, and tends to be more remarkable with a less dose of radiation. Consequently, the false image remarkably appears in a portion of the image, such as a bone, where a dose of radiation is likely to lack.
The statistical noise appears not only in the bone portion but also the entire image. Thus, a construction has been conventionally designed that removes the false image derived from the statistical noise from the image to enhance visibility of the image. See, for example, Japanese Patent No. 4072491 and Japanese Patent Publication 2004-242285A. Such the image processing has a problem how an original subject image is allowed to remain while the false image is removed.
Hereinunder described are two sophisticated examples of the image processing with the conventional construction. With a method in Japanese Patent No. 4197392, edge reliability is firstly calculated for each pixel. The edge reliability is an index representing whether a variation in pixel value between a pixel constituting the image and a pixel surrounding the pixel is derived from a configuration of the subject or from the statistical noise. Then, different weights are applied to each of the pixels in accordance with the calculation results to perform image processing. Specifically, a weight of an anisotropic filter image is increased for a pixel with high edge reliability and pixel values variation due to the configuration of the subject, the anisotropic filter image being obtained by applying an anisotropic filter to an image to perform smoothing depending on a direction. On the other hand, a weight of a band image is increased for a pixel with low edge reliability and pixel values variation due to the statistical noise. This allows removal of the false image derived from the statistical noise in the image while the configuration of the subject remains.
Here, an edge indicated by the edge reliability is a streak figure derived from the configuration of the subject. Since the false image derived from the statistical noise is granular, the false image has no streak appearing therein. Accordingly, a streak figure in the image does not represent the statistical noise but represents the configuration of the subject. Thus, the streak configuration derived from the subject appears in a portion of the image having high edge reliability. As a result, an anisotropic filter is applied to the streak configuration.
In this case, the anisotropic filter is applied so as to smooth along the streak configuration. A direction of the anisotropic filter, i.e., a direction of the streak configuration is accurately calculated with suppressed noise influence by averaging gradient vectors adjacent to one another isotropically and correcting the vectors with a gradient vector with low resolution. This avoids blurring of the streak configuration itself while the statistical noise superimposed on the streak configuration is eliminated.
Next, description will be given of an image processing method in Patent Literature 4. In this method, an anisotropic filter is variable in accordance with the edge reliability even in the same direction. That is, an anisotropic filter with high direction dependency is applied when the edge reliability is high. This leads to image processing to the configuration in the image with remarkable streaks so as to remain the configuration, and leads to image processing to the blurred configuration in the image so as to remain the blurring.
Patent LiteraturePatent Literature 1Japanese Patent No. 4072491Patent Literature 2Japanese Patent Publication No. 2004-242285APatent Literature 3Japanese Patent No. 4197392Patent Literature 4Japanese Patent Publication No. 2002-133410A